We’ve all done it, at some time or other. You’re doing something in one room, when it suddenly occurs to you that you need to go to a different room to get something. Along the way, something distracts you—a tune on the radio, someone saying something amusing as you pass, or suddenly figuring out a plot twist in a TV show that’s been bugging you for months. Whatever it is, you reach your destination and suddenly have no idea why you decided to go there. It’s frustrating, it’s annoying, it’s time-wasting; it’s one of the many quirks thrown up by the surprisingly complex way the brain processes memory.
The most familiar division in human memory for most people is that between short-term memory and long-term memory. These differ considerably, but are still interdepen-dent. Both are appropriately named; short-term memories last about a minute, max, whereas long-term memories can and do stay with you your whole life. Anyone referring to something they recall from a day or even a few hours ago as “short-term memory” is incorrect; that’s long-term memory.
Short-term memory doesn’t last long, but it deals with actual conscious manipulation of information; the things we’re currently thinking about, in essence. We can think about them because they’re in our short-term memory; that’s what it’s for. Long-term memory provides copious data to aid our thinking, but it’s short-term memory that actually does the thinking. (For this reason, some neuroscientists prefer to say “working” memory, which is essentially short-term memory plus a few extra processes, as we’ll see later.)
It will surprise many to find that the capacity of short-term memory is so small. Current research suggests the average short-term memory can hold a maximum of four “items” at any one time.1 If someone is given a list of words to remember, they should be able to remember only four words. This is based on numerous experiments where people were made to recall words or items from a previously shown list and on average could recall only four with any certainty. For many years, the capacity was believed to be seven, plus or minus two. This was labeled as the “magic number” or “Miller’s law” as it was derived from 1950s experiments by George Miller.2 However, refinements and reassessment of legitimate recall and experimental methods have since provided data to show the actual capacity is more like four items.
The use of the vague term “item” isn’t just poor research on my part (well, not just that); what actually counts as an item in short-term memory varies considerably. Humans have developed strategies to get around limited short-term-memory capacity and maximize available storage space. One of these is a process called “chunking,” where a person groups things together into a single item, or “chunk,” to better utilize their short-term memory capacity.3 If you were asked to remember the words “smells,” “mom,” “cheese,” “like,” and “your,” that would be five items. However, if you were asked to remember the phrase “Your mom smells like cheese,” that would be one item, and a possible fight with the experimenter.
In contrast, we don’t know the upper limit of the long-term-memory capacity as nobody has lived long enough to fill it, but it’s obscenely capacious. So why is short-term memory so restricted? Partly because it’s constantly in use. We’re experiencing and thinking about things at every waking moment (and some sleeping ones), which means information is coming and going at an alarmingly speedy rate. This isn’t somewhere that’s going to lend itself well to long-term storage, which requires stability and order—it would be like leaving all your boxes and files in the entrance of a busy airport.
Another factor is that short-term memories don’t have a “physical” basis; short-term memories are stored in specific patterns of activity in neurons. To clarify: “neuron” is the official name for brain cells, or “nerve” cells, and they are the basis for the whole nervous system. Each one is essentially a very small biological processor, capable of receiving and generating information in the form of electrical activity across the cell membranes that give it structure, as well as forming complex connections with other neurons. So short-term memory is based on neuronal activity in the dedicated regions responsible, such as the dorsolateral prefrontal cortex in the frontal lobe.4 We know from brain scanning that a lot of the more sophisticated, “thinking,” stuff goes on in the frontal lobe.
Storing information in patterns of neuronal activity is a bit tricky. It’s a bit like writing a shopping list in the foam on your cappuccino; it’s technically possible, as the foam will retain the shapes of words for a few moments, but it doesn’t have any longevity, and hence can’t be used for storage in any practical sense. Short-term memory is for rapid processing and manipulation, and with the constant influx of information anything unimportant is ignored, and quickly overwritten or allowed to fade away.
This isn’t a foolproof system. Quite often, important stuff gets bumped out of short-term memory before it can be properly dealt with, which can lead to the “Why did I just come in here?” scenario. Also, short-term memory can become overtaxed, unable to focus on anything specific while being bombarded with new information and demands. Ever seen someone amid some hubbub (such as a children’s party, or a frantic work meeting) with everyone clamoring to be heard, suddenly declare, “I can’t think with all this going on!”? They’re speaking very literally; their short-term memory isn’t equipped to cope with that workload.
Obvious question: if the short-term memory where we do our thinking has such a small capacity, how the hell do we get anything done? Why aren’t we all sitting around trying and failing to count the fingers on one hand? Luckily, short-term memory is linked to long-term memory, which takes a lot of pressure off.
Take a professional translator; someone listening to long detailed speech in one language and translating it into another, in real time. Surely this is more than short-term memory can cope with? Actually, it isn’t. If you were asking someone to translate a language in real time while actually learning the language, then, yes, that would be a big ask. But for the translator the words and structure of the languages are already stored in long-term memory (the brain even has regions specifically dedicated to language, like Broca’s and Wernicke’s areas, as we’ll see later). Short-term memory has to deal with the order of the words and the meaning of the sentences, but this is something it can do, especially with practice. And this short-term/long-term interaction is the same for everyone; you don’t have to learn what a sandwich is every time you want a sandwich, but you can forget that you wanted one by the time you get to the kitchen.